Historically, generation of ions for mass spectrometric analysis has required ionisation of a sample in a vacuum or near vacuum.
More recently various techniques have been developed which enable a sample to be ionised at or near atmospheric pressure, which significantly increases the utility of mass spectrometric analysis. One such technique is known as DART (direct analysis in real time). With DART a neutral carrier gas, typically Helium, is ionised at atmospheric pressure by a kilovolt electrical discharge and the ionised gas is directed on to a sample to be analysed in order to ionise the sample. Ionisation of the sample occurs at atmospheric pressure via a series of competing reactions beginning with Pennington ionisation in which a long-lived (Metastable) excited state Helium molecule induces an energy transfer to the sample resulting in formation of a radical ion. This ionisation is arranged to take place in a small gap between the source of ionised Helium and the inlet to a mass spectrometer so that sample ions are drawn into the mass spectrometer for analysis.
A drawback with the DART technique is the need to use a costly carrier gas such as Helium. Also, high electrical potentials are required to ionise the carrier gas at atmospheric pressure which, in some implementations, may run the risk of exposing those high potentials to users. There is also a need to position a sample for analysis suitably in relation to the ionised gas source and inlet to the mass spectrometer, which can be inconvenient.
Embodiments of the present invention have been made in consideration of these problems.